PHASE BEHAVIOUR OF THE DIOCTADECYLDIMETHYLAMMONIUM BROMIDE WATER SYSTEM

The phase behaviour of the twin-tailed surfactant dioctadecy1dimethyla mmonium bromide with water was studied by DSC, FT-IR, X-ray and polari zing microscope. The phase diagram of DODAB-water system is very simil ar to that of DODAC-water. The dihydrate is in equilibrium with isotro pic solution below 55-degrees-C. Above this temperature there is a lam ellar liquid crystalline region, in equilibrium with isotropic liquid and solid crystals of DODAB.2H(2)O, up to 69-degrees-C. From 69 to 86- degrees-C, the lamellar mesophase is in equilibrium with 'waxy' anhydr ous DODAB. From 86 to 116-degree-C and very high DODAB content, there is a very narrow region of existence of inverse hexagonal mesophase, i n equilibrium via a narrow biphase region with lamellar mesophase. At T>116-degrees-C an isotropic liquid appears. There seems to exist two different lamellar mesophases, one of them between 10 and 40 wt.% DODA B and the other between 60 and about 97 wt.% DODAB, with a biphase zon e between them.